secular variation

Secular variation refers to the continuous, short-term changes observed in Earth's magnetic field. These variations are distinct from longer-term geologic processes. Observations in London since 1540, for instance, show the magnetic field's direction has nearly completed a full cycle over that period.12

Maps of secular variation reveal regions where specific components of the field are increasing or decreasing, with changes potentially reaching 150 nanoteslas per year. Many of these features appear to drift westward over time, suggesting a connection to the dynamo mechanism within Earth's liquid outer core. The exact origin of this secular variation is still a subject of research in geophysics.123